This invention pertains to signal spectrum equalizers of the type that provide adjustment in incremental bandwidths with a minimum of interaction between each spectrum segment. While this invention has general applicaion, the principles it embodies are especially suited for graphic and parametric equalizers used in performing real, or near real-time audio spectral manipulation with minimum operator effort. Such efficient spectrum manipulation is essential in the recording and broadcast industries and in environmental conditions for the performing arts.
Equalizers in current use fall into two broad categories: parametric (also known as "variable parameter") and graphic.
Parametric. The parametric or varible parameter equalizer consists of a number of bandpass filter sections operating independently with respect to each other within the passband of the equalizer. Each filter section permits independent adjustment of center frequency, bandwidth and level.
Graphic. The ideal graphic equalizer permits a one-to-one correspondence between the position of each filter segment control (the "slider") and a graph of the desired equalizer level adjustment vs. frequency. Each filter segment has a fixed center frequency. This permits rapid and unambiguous filter adjustments with a minimum of ancillary test equipment.
Although not as well known, there also exists a hybrid of the above two major classes. This hybrid is a specially designed graphic equalizer that permits some adjustment of the frequency centers by an interpolation effect, hence called the "interpolating" graphic equilizer.
The design and use of these equilizers depends in part on the standard center frequencies that have been prescribed by the International Standards Organization (ISO) for the audio equalizers and other equipment where fixed frequency characterization is required. While the ISO standards are generally useful for many applications, other uses require complete discretion in center frequency selection. An example of this need is in the characterization and modification of the reverberation spectrum of a performance hall where frequencies falling in between ISO allocations may occur. The parametric equalizer provides this flexibility, but economic considerations severely limit the number of parametric filters required to cover the complete spectrum. The interpolation filter provides a compromise between parametric control and the number of required filter sections, as well as the convenience of a graphics format.
These families of equalizers have a common design problem. That is the achievement of constant bandwidth with independent changes in "boost" (gain) and "cut" (attenuation) for each bandpass filter. This problem, which is a characteristic of most commonly used equalizer designs, is illustrated in the graph of FIG. 5 that accompanies the detailed description below. For the various boost levels chosen in FIG. 5, it is clear that the bandwidth changes by a large percentage. This interdependency of bandwidth and gain prevents the amplitude selectivity of the filter from being faithfully implemented as one of a number of filter segments in the equalizer, except at the levels near maximum boost or cut. Therefore, the parameters of the parametric equalizer become extremely interdependent. This interdependency in turn lessens the ability of the interpolating equalizer to achieve smooth transition to intermediate signal frequencies between adjacent filters, i.e., low ripple factor. It also degrades the desirable match between the levels of equalizer response, as indicated by each slider position of the graphics equalizer, and a given spectrum response chart.
If, on the other hand, the bandpass filters were interconnected so as to exhibit a "constant Q", the above interdependency problems of equalizer designs would be greatly alleviated. The term "constant Q" refers to the ability to achieve a constant percentage bandwidth for a given bandpass center frequency. FIG. 4 of the accompanying drawings illustrates the frequency response characteristics of a bandpass filter at several different boost levels for a filter segment employing this "constant Q" feature. It is apparent that the effective bandpass characteristics of the basic filter are maintained over the indicated levels of gain.
Another feature often required in the designs of these equalizers is that of spectral symmetry of the boost compared to cut modes. Symmetry facilitates the synthesis of an inverse transfer function, thus providing a complementary phase and amplitude characteristic useful for altering or "flattening" a given frequency response profile. In FIG. 6 of the drawings that accompany the detailed description, an equalizer response is shown having this boost-cut symmetry. However, in some applications it may be required or desirable to retain an asymmetrical spectral shape. An example of the latter is where the cut mode has certain rejection characteristics that can be positioned at selected frequency points to compensate for undesirable environmental acoustics. The diagram in FIG. 7 illustrates this asymmetrical feature. An asymmetrical boost-cut response is known from the filter design work of Van Ryswyk et al disclosed in U.S. Pat. No. 3,755,749; but that configuration has the drawback of providing only one frequency band per section.
Another desirable aspect of equalizers is that of providing "fail-soft" characteristics in the overall design. Some configurations use tandem filter sections which have serious single point failure charcteristics, i.e., any single filter failure would disable the operation of the entire array of filters. For parallel filter architectures, a filter failure would result in undesirable "holes" in the spectrum envelope. This problem was first addressed by Gundry in U.S. Pat. No. 3,921,104, who provides a design in which failure of a filter section results in the filtered output spectrum segment reverting to the benign values of the input spectrum, thus "failing-soft".
These and other design considerations and certain proposed solutions are set forth in my article "Constant-Q Graphic Equalizers", J.. Audio Engineering Society, Vol. 34, No. 9, 1986 September, pages 611-626.